1. FIELD OF THE INVENTION
The present invention relates to gas assisted injection molding and to a gas control system for use in a gas assisted injection molding system. More particularly, the invention relates to a method and apparatus for gas assisted injection molding which has the ability to inject a shot of molten plastic into an injection mold, shut off the plastic flow, and inject a gas at a first predetermined and precisely control pressure into the melt, either almost instantaneously or after a delayed time. The present invention then has the ability to step up and/or step down and/or hold the gas under pressure in an almost infinite variety of patterns while the plastic is cooling in the mold, finally arriving at a step down or controlled vent pressure when the part is sufficiently cooled to be self-supporting, followed by opening the mold and removing the article.
2. DESCRIPTION OF THE PRIOR ART
The history of injection molding as a manufacturing process is relatively recent compared to some other manufacturing methods. In the infancy of injection molding, articles were generally produced by injecting a molten plastic or resin material into a mold cavity, and letting the material cool to form a molded article, at which time the mold cavity was opened and the article was released.
As the art of conventional injection molding advanced, attempts were made to make larger and larger parts. It become evident that there were many restrictions on the ability of conventional injection molding in some of these applications. Some restrictions which were found the impossibility to match, while molding a part, different factors as the presence of large, flat surfaces, diverse and heavy wall sections and exceptionally long flows with the need for acceptable physical properties, good productivity and investment levels, and an aesthetically pleasing surface finish.
These considerations were found to be particularly relevant in the electronics industry, where there was the need for enclosures for television sets, computers, printers and the like; in the home furnishings industry, where there was a need for large parts for garden chairs and tables, bathroom furnishings, and the like. Also presenting a problem were tools, handles and machinery components in general, and in the automobile industry in particular. A particularly aggravating problem was the tendency of sink marks to appear opposite large or thick rib sections. This led those skilled in the art to devise a method and apparatus capable of generating a pressure inside the molded component in addition to the injection pressure exerted by the machine at the injection point.
One solution that proved satisfactory for some injection molded parts was the use of structural foams. Processes for structural foam molding date back to the 1960s. In the structural foam molding process, a gas and polymer mixture is injected inside the plasticizing barrel on the nozzle of the injection molding machine. Once injected, the gas expands and forms cavities which are generally closed cells in which gas is entrapped. The structural foam injecting process reduces internal stresses and surface sink marks, and permits a reduction in the weight of the material used. However, for some applications, it was found that gas bubbles could migrate to the surface and burst therethrough, resulting in the unsuitability of the foam process for certain parts. Also, the cycle time is longer and special molds need to be used. Therefore, the foam injection molding process remained unsatisfactory for many applications, and those in the art continued their search to find a satisfactory method of injection molding for use on large and/or complicated parts.
In order to solve the problems in the injection molding art which could not be solved with structural foam molding and other methods, several in the art developed the process of gas assisted injection molding. In gas assisted injection molding, unlike structural foam molding, the cavities formed in the molded component remain in connection with the injection point of the gas. In such a process, the mold is filled with molten plastic or synthetic resin molding material which, in many processes, is less than the volume which is necessary to fill the mold. The filling of the mold is completed by the injection of a gas into the molding material contained in the mold. This results in the formation of a cavity or cavities within the molding material. The pressure of the injection gas compresses or packs the melt against the mold walls during the curing state, resulting in the elimination of sink marks opposite thicker sections or ribs and the like.
Many variations of the gas assisted injection molding process are present in the prior art. For example, U.S. Pat. No. 4,106,617 requires a "sprue break" for venting the gas out of the part before opening the mold. U.S. Pat. No. 4,106,887 requires gas to be injected through a needle valve within a runner which is movably mounted and engagable in the cavity gate. U.S. Pat. No. 4,140,672 provides for the injection of viscous liquid which, at room temperature, is either a grease or solid.
U.S. Pat. Nos. 4,136,220 and 4,247,515 relate to the formation of a structural web type material. U.S. Pat. No. 4,498,860 relates to a movable ram associated with the cavity gate which is used to cut off the sprue. U.S. Pat. Nos. 4,830,812 and 4,913,644 relate to cutting members within the mold body that are used to vent the pressurized gas. U.S. Pat. No. 4,923,666 relates to a method wherein the cavity is fully packed with resin before gas injection, and gas cavities are formed only in areas in which there is significant shrinkage.
U.S. Pat. No. 4,923,677 requires a gas venting passage separate from either the resin or gas injection passages. U.S. Pat. No. 4,943,407 requires that gas injection be through the sprue bushing. U.S. Pat. No. 4,944,910 requires a simultaneous injection of the resin and gas during the molding cycle, while U.S. Pat. No. 4,948,547 requires that the injection gas is confined to the resin flow path, and that it does not enter the article defining area. And finally, U.S. Pat. No. 5,028,377 requires a flow path at least partially defined by a movable member wherein the pressure of the molten plastic can be reduced in a controlled manner by moving the movable member prior to gas injection.
Also, various types of control systems have been developed in the prior art to control the pressure of the injection gas. U.S. Pat. No. 4,824,732 requires a variable volume chamber for gas storage and control. The gas is injected at a controlled rate by reducing the volume of the variable volume chamber. U. S. Pat. No. 4,855,094 requires the injection of the gas at a predetermined pressure no greater than the resin injection pressure. U.S. Pat. No. 4,935,191 requires that both the resin and the gas be injected simultaneously for at least a portion of the injection cycle. And U.S. Pat. No. 5,015,166 provides for a control system for supplying a variable volume of a gas art at a predetermined pressure and maintaining that pressure at a substantially constant level. All of these patents perform generally satisfactorily, but leave one or more problems in the injection molding art. Thus, those skilled in the art continued their search for a satisfactory gas assisted injection molding method and apparatus.
Applicants have found, as more fully explained in co-pending patent application Ser. No. 07/628,746, filed on Dec. 17, 1990 and entitled "Method and Apparatus for Gas Assisted Injection Molding", that starting a flow of injection gas during the time that a flow of molten material is taking place has caused the problem of clogging of the gas supply passages due to the molten material entering and clogging the supply passage during the initial injection of molten plastic. This has involved costly down time and complicated apparatus to try and prevent the entry of the molten molding material into the gas supply system. In patent application Ser. No. 07/628,746, which is specifically incorporated herein by reference, it is shown that it is possible to sequentially first introduce a molding material into the mold cavity, and then shut off the supply of plastic. Thereafter, pressurized gas is introduced through the injection nozzle into the interior of the mold cavity, producing a satisfactory solution to the problems in the prior art.
Further work by the Applicants since the filing of patent application Ser. No. 07/628,746 has shown that it is also possible to eliminate problems and to provide a higher quality part by providing a method and apparatus which produces precise and careful control of the pressure of the injection gas during injection into the part and during the cooling process. The method and apparatus for precision control of the injection gas is preferably used in Applicants' system, wherein the gas injection is not started until after the supply of molten resin or plastic into the mold is stopped. It should be understood, however, that such system can also be used to improve the performance of other prior art gas assisted injection molding systems made by others.